1. Field of the Invention
The present invention relates to a method of measuring alignment of a measurement pattern used for manufacturing a semiconductor device and, especially, to a method of measuring alignment of a measurement pattern relative to a pattern formed in a preceding process after a resist pattern is formed for photolithography.
2. Description of the Related Art
FIG. 8 shows the principal of alignment measurement according to the conventional method. In FIG. 8, appropriate regions are selected from a preceding pattern 10 formed in the preceding process, for example, a box-shaped pattern 10 and a present pattern 100 being processed at present, for example, a box-shaped pattern 100, so that symmetric waveforms are obtained from the regions by the waveform treatment. The waveforms of the patterns 10 and 100 are recognized or detected and graphically treated for linear approximation to obtain the peak points of the preceding and present processes or steps. That is, a resist pattern for alignment measurement comprises the preceding box pattern 10 provided at an outermost position and the present box pattern 100 provided inside the box pattern 10 with a predetermined space.
Pattern recognition elements, for example, photo-sensors 100-104 are arranged along a section A-A′, which is selected as an appropriate region to obtain symmetric waveforms of the positive type box patterns of the preceding and present processes. The waveform signals of line patterns at the section A-A′ are treated to obtain a characteristic B1-B1′. The characteristic B-B′ is treated in an alignment measuring apparatus to obtain a characteristic B2-B2′. That is, the line pattern containing edges 10-1 and 10-2 becomes the characteristic B1-B1′ containing points 10-3, 10-4, and 10-5 as a result of the treatment of the waveform signals and then, becomes the characteristic B2-B2′ containing points 10-6, 10-7, and 10-8 as a result of the waveform treatment in the alignment measuring apparatus.
Consequently, the line pattern containing the edges 10-1 and 10-2 is characterized in that the concentration value thereof increases linearly up to the point 10-8 from the edge points 10-6 and 10-7. The point 10-8 represents the central concentration value of the line pattern. The center of the characteristic pattern obtained by the waveform treatment of the line pattern, such as the point 10-8, is referred to as a “central point”.
In the same way, the box pattern 100 containing edges 100-1 and 100-2 is changed to the characteristic B1-B1′ containing points 100-3 and 100-4 by the waveform treatment at the section A-A′ and then, changed to the characteristic B2-B2′ containing points 100-5 and 100-6 by the waveform treatment in the alignment measuring apparatus. That is, the edge 100-1 becomes the point 100-5 as a result of the waveform treatment at the section A-A′.
The alignment measurement is performed by measuring at least one place, such as an interval between the points 10-8 and 100-5. In FIG. 8, two intervals are measured as shown by arrows Z in the characteristic B2-B2′. It is possible to select any place for X-direction and Y-direction measurements as long as symmetric waveforms are obtained. It is not necessary to measure in both the X and Y directions at each measurement place and any combination of the X and Y direction measurements at different places is acceptable.
As a microscopic pattern is developed, a resist pattern (hole pattern) produced by the ordinary KrF exposure/development method is subject to a baking process of high temperature to generate a heat flow in the resist pattern so that the internal diameter of the hole pattern is reduced when the resist pattern is shrunk by the heat flow.
FIG. 10 shows the principal of pattern shrinkage by the heat flow according to the conventional method. A hole H having a circular section is provided in a resist pattern. The hole H before the heat flow shown on the left-hand side becomes a hole H′ shown on the right-hand side after the heat flow. An internal diameter a of the hole H is reduced by the heat flow to an internal diameter a′ of the hole H′. This method makes it possible to manufacture a pattern of 0.10 μm or less, which is higher than the resolution limit by the KrF exposure technology.
The method of reducing the hole diameter by the heat flow, however, has the following problems.
(1) If the resist pattern has a very large dimension before the heat flow, for example, if the hole internal diameter is 0.5 μm or more, the resist pattern deteriorates. That is, since the thickness of a resist film is substantially constant, the thickness b of the film at a linear section is reduced due to the increased amount of resist flown in the hole when the hole internal diameter is larger than a certain value, thus causing adverse effects on the etching process after the photolithography.
(2) How the pattern or hole is shrunk is dependent on other holes exist on the left and right-hand sides of the hole. That is, the form of the shrinkage by the heat flow is varies with the amount of the resist flown in the periphery of the hole. Where the holes exist densely, the resist amount per hole is small, which reduces the shrinkage of the pattern. This mechanism is described with aspect to FIG. 11.
FIG. 11 shows the symmetric character of the heat flow according to the prior art. A resist pattern having holes provided at the same interval and having the same diameter is heat-shrunk as described below. A hole A is uniformly shrunk on the upper, lower, and right sides because of the presence of other holes, while the hole A is shrunk to a larger extent on the left side because of the larger amount of resist flow-in due to absence of other holes. Accordingly, the center of the hole A moves to the side of a hole B after the heat flow.
The hole B is uniformly shrunk on every side because of the presence of other holes. A hole C is uniformly shrunk on the upper, lower, and left sides because of the presence of other holes, while the hole C is shrunk to a larger extent on the right side because of larger amount of flow-in resist due to absence of other holes. Consequently, the center of the hole C moves to the side of the hole B after the heat flow.
(3) A fine hole is not sufficiently shrunk unless the width of a space between resist holes is greater than twice that of the resist holes before the heat flow. It is proved from the above-mentioned fact that unless holes are spaced from each other to a certain extent, the holes are not sufficiently shrunk and the sufficient height of the linear section of the resist is not obtained due to a small amount of flow-in resist. Also, experiments show that desired characteristics are obtained when the width of a space between holes is greater than twice that of the holes.
The results of the experiments are as follows:
(1) Object of the Experiments
To measure the conditions under which the thickness b of the linear section in FIG. 10 becomes sufficiently practical after the heat flow.
(2) Conditions
Resist: TDUR-P015film thickness of 10,000 ÅReflection preventive filmFilm thickness of 1,100 Å(bark material): SWK-EX2NSG filmfilm thickness of 10,000 ÅWafer: Si-substrateExposure energy85 mjManufacturing methodheat shrinkage(3) Layer Structure
TDUR-P015/SWK-EX2/NSG film/Si-substrate
(4) Results
When the hole diameter of a resist mask is fixed at 0.26 μm, the samples having a hole pitch of 0.52 μm or more satisfied the above conditions. The samples having a hole pitch of 0.78 μm or 1.04 μm also satisfied the above conditions.
(4) Also, a large resist pattern, such as a pattern for alignment measurement, loses the linearity of pattern edges after the heat flow. That is, every resist pattern in a wafer is shrunk as shown in FIG. 10 regardless of the pattern size thereof since the heat flow is produced in the entire wafer. The pattern for alignment measurement requires a relatively large size because the alignment measurement is optically performed. For a large pattern, as shown in FIG. 11, the amount of flow-in resist varies with the position thereof. For example, in the present pattern 100 in FIG. 9, the central portion of each side thereof undergoes the largest drift because of the largest amount of flow-out resist. Consequently, each side of the pattern is curved.
FIG. 9 shows a resist pattern for alignment measurement after the heat flow according to the prior art. Unlike the box pattern 100 in FIG. 8, the present box pattern 100 is shrunk, when the heat flow treatment is applied. When the resist pattern after the heat flow is subject to the alignment measurement, the waveforms of preceding pattern or box 10 and the present pattern or box 100 are processed to obtain the central points of the patterns.
More specifically, the preceding box pattern 10 is provided at the outermost side, and the present box pattern 100 is provided inside the box pattern 10 at a predetermined interval. The photo-sensors 101-104 for pattern recognition are arranged along the section A-A′. The line patterns of the preceding positive type box pattern and a plurality of the present positive type patterns at the section A-A′ are processed to obtain the characteristic B1-B1′, which in turn is processed in an alignment measuring apparatus to obtain the characteristic B2-B2′.
That is, the line pattern containing edges 10-1 and 10-2 becomes the characteristic B1-B1′ containing points 10-3, 10-4, and 10-5 as a result of the process of the waveform signal, and then, the characteristic B-B′ is turned to the characteristic B2-B2′ containing points 10-6, 10-7, and 10-8 by the waveform-treatment in the alignment measuring apparatus. Consequently, the line pattern at the section A-A′ has the characteristic that the concentration value increases linearly up to the point 10-8 from the edge points 10-6 and 10-7. The point 10-8 represents the central concentration value of the center of the line pattern.
As shown in FIG. 9, the resist pattern or box is shrunk by the heat flow caused by a high temperature baking process so that the linearity of the pattern edges is not maintained. That is, the sides of the box pattern under measurement are curved, forming arcs 120. Curved edges 120-1 and 120-2 of the box pattern 100 are turned to points 120-3 and 120-4 in the characteristic B1-B1′ by the waveform treatment at the section A-A′, and then, turned to points 120-5 and 120-6 in the characteristic B2-B2′ by the waveform treatment in the alignment measuring apparatus. Consequently, the edge 120-1 becomes the point 120-5. The measurement of an interval between the points 10-8 and 120-5 represents the measurement of the position at which the present box pattern 100 is curved and deformed by the heat flow.
In FIG. 9, a comparison between the characteristic B1-B1′ and the characteristic B2-B2′ shows that the shrinkage of the resist pattern edge by the heat flow is a real problem. Especially, when the pattern edges are measured on both the left and right-hand sides, an asymmetric waveform signal is produced by the heat flow. When measured, the asymmetric waveform produces a deviation between the measured position and proper position of the central point, thus producing an adverse effect on the value of the alignment measurement.
As described above, the linearity of the present resist pattern is broken by the heat flow. Consequently, the asymmetric waveform signal is processed, thus shifting the central position of the present pattern, causing an error in the alignment measurement.